The present invention relates to a self substrate bias generator and, more particularly, to a self substrate bias generator for use in a large scale integrated circuit.
A self substrate bias generator for maintaining the potential of a P-type semiconductor substrate to a predetermined potential below the earth potential (VSS) which is applied to the circuit formed on the semiconductor substrate is known. This substrate voltage generator is formed as shown in, for example, FIG. 1A. In FIG. 1A, an output terminal of an oscillator 1 is connected through a capacitor C1 to the gate of a transistor T1, to one end of the current path of transistor T1, and to the cathode of a diode D1. The output terminal of oscillator 1 is also connected through an inverter 2 and a capacitor C2 to the gate of a transistor T2, to one end of the current path of transistor T2, and to the cathode of a diode D2. The earth potential (VSS) of the circuit formed on this substrate is supplied to the other ends of the current paths of transistors T1 and T2. The anodes of diodes D1 and D2 are connected to the semiconductor substrate. As diodes D1 and D2 in the circuit shown in FIG. 1A, the PN junctions between N.sup.+ (N-type high concentration) layers serving as the drains of transistors T1 and T2 and a P-type semiconductor substrate (P-SUB) are generally used as shown in a cross sectional view of FIG. 1B.
The potential of nodes Qi (i=1 or 2) shown in FIG. 1A becomes an L level in response to a clock signal from oscillator 1 and the potential of a node Pi shown in FIG. 1A is reduced through capacitor Ci. When the potential of node Pi decreases, diode Di is turned on and the charges in the semiconductor substrate are pumped to node Pi. When the potential of node Pi becomes an H level, the charges pumped to node Pi are pumped to earth potential VSS by transistor Ti. Potential VBB of the semiconductor substrate is maintained to below earth potential VSS by the above series of operations. To efficiently use oscillator 1, two sets, each consisting of capacitors C1, C2, diodes D1, D2, and transistors T1, T2, respectively, are used. Each set operates independently.
The foregoing circuit has the following two drawbacks:
(1) The first drawback relates to transistor Ti. When transistor Ti pumps the charges which have previously been pumped to node Pi to earth potential VSS, transistor Ti operates in a pentode operation. However, the efficiency when transistor Ti pumps the charges by the pentode operation is lower than the efficiency when transistor Ti pumps the charges by the triode operation. On the other hand, the potential of node Pi decreases to the level of only VSS+VT because of the threshold voltage VT of transistor Ti. Therefore, the charges which are pumped from node Pi to potential VSS decrease by an amount of voltage VT, so that the charges which are pumped from the substrate to potential VSS are reduced. To avoid the decrease in charges which are pumped, the method whereby the gate of transistor Ti is pulled up is also used. However, in this case, the circuit construction becomes remarkably complicated.
(2) The second drawback relates to diode Di. When diode Di pumps the charges in the semiconductor substrate to node Pi, a number of minority carriers (electrons) are injected into the semiconductor substrate. The life time of the electrons is fairly long and when the potential of node Pi becomes an H level and the potential of the N.sup.+ layer in FIG. 1B becomes high, the injected planted electrons flow back into the N.sup.+ layer. Namely, the N.sup.+ layer of transistor Ti connected to node Pi operates in a manner similar to the guard ring and collects the electrons. This operation intends to again attract the electrons which were injected into the substrate. The efficiency (pumping efficiency) of the substrate voltage generator for pumping the charges in the substrate to potential VSS deteriorates remarkably. According to the experiments by the applicant, it has been found that the pumping efficiency was reduced to about 1/4 as compared with the case where the electrons are not collected again. To maintain constant pumping performance, it is necessary to enlarge the dimensions of the whole circuit such as capacitor Ci, transistor Ti, oscillator 1, and the like.
In addition, there is the undesirable possibility that the injected electrons may exert an adverse influence on the operation of the electronic circuit formed on the semiconductor substrate. Practically speaking, in the case where the circuit formed on the semiconductor substrate is a dynamic memory, there is a potential for destroying the stored data. For example, it is now assumed that the positive charges are accumulated in the memory cell. These positive charges attract the injected electrons and are coupled with the electrons. The accumulated positive charges gradually decrease and the stored data is destroyed.
Similar disadvantages also occur when the conventional substrate voltage generator is formed on the N-type semiconductor substrate.